The invention is directed to a rotary drier and specifically a drum asphalt plant which includes a rotatable drum into which is fed virgin aggregate, reclaimed asphalt and liquid asphalt which are admixed and heated to produce a finished asphalt (pavement) composition. The invention is specifically directed to a counterflow drier or a so-called double shell drum mixer in which the virgin aggregate and the reclaimed asphalt (millings, fines and sand) travel in a direction opposite to the direction of the hot gases normally generated by a combustion unit which burns natural gas or oil. The burner flame creates an extremely hot zone in the area of its discharge from the burner tube and the temperature progressively decreases toward the end of the drum into which the coarse aggregate and reclaimed asphalt is fed into the drum. At the discharge end of the drum which is downstream from the high temperature zone and essentially downstream and behind the burner flame, the temperature is appreciably reduced. Accordingly, in such asphalt plants there is an undesirable high temperature zone generally adjacent the burner flame, and an equally undesirable low temperature zone downstream of the burner flame at the discharge end of the drum. Desirably, a uniform temperature throughout the drum is optimally required, though practically and technically impossible, yet at a minimum it is highly desirable to heat the discharge end of lhe drum and, as best as possible, minimize extremes of high and low temperature zones.
In U.S. Pat. Nos. 3,845,941 issued Nov. 5, 1974 and 4,000,000 issued Dec. 28, 1976 in the name of Robert L. Mendenhall, efforts have been made at achieving uniformity of drum temperature by utilizing tubes positioned lengthwise of the drum through which hot air is conducted. However, in each of these drums the highest temperature is immediately at the discharge end of the drum which is undesirable, while the lowest temperature is at the entrance end of the drums. Because of this an excessive amount of heat/BTU's is introduced into the asphalt composition virtually immediately upon its discharge and thus is wasted whereas this heat/BTU's should more desirably be utilized initially to heat the incoming virgin aggregate and reclaimed asphalt. Accordingly, though uniformity of drum temperature is obviously desirable, such is not achieved, and whatever heat distribution is effected by the drums of these patents, such is obtained through excessively high heating of the asphalt composition/mixture immediately upon its discharge.